[evolution/gnome-3-28] Avoid runtime warning when printing message with text/calendar attachment

[Milan Crha <mcrha src gnome org>]

commit 77ea3599406669019463a2e402b172cc2f8332f0
Author: Milan Crha <mcrha redhat com>
Date:   Fri Mar 23 11:58:50 2018 +0100

    Avoid runtime warning when printing message with text/calendar attachment
    
    Discovered as part of the bug 792688.

 src/modules/itip-formatter/e-mail-formatter-itip.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/src/modules/itip-formatter/e-mail-formatter-itip.c 
b/src/modules/itip-formatter/e-mail-formatter-itip.c
index a2d10c8..551a577 100644
--- a/src/modules/itip-formatter/e-mail-formatter-itip.c
+++ b/src/modules/itip-formatter/e-mail-formatter-itip.c
@@ -59,7 +59,10 @@ emfe_itip_format (EMailFormatterExtension *extension,
        GString *buffer;
        EMailPartItip *itip_part;
 
-       g_return_val_if_fail (E_IS_MAIL_PART_ITIP (part), FALSE);
+       /* This can be called with attachment parts too, thus
+          return silently in that case */
+       if (!E_IS_MAIL_PART_ITIP (part))
+               return FALSE;
 
        itip_part = (EMailPartItip *) part;